They were initially built for lower income people close to public transportation or even walking distance to work. None ever had enough yard space to build a barn for horses and wagons.

Having lived in one, I can say that the biggest drawback was that you could hear the person above you walking around, and if you lived on the third floor, carrying a couple of grocery bags up 3 flights of stairs got old quickly. The next biggest drawback is there's typically no space for expansion as your family grows. Some people combine two apartments into one, but that's awkward.

Bottom line is, there's a crisis in NH of not enough housing and three deckers seem like an ideal solution.

It's also good for a first house with tenants paying your mortgage for you and tenants having quick access to the landlord.

Something else that no one mentions; having 3 apartments is great for inlaws so that you don't have them sharing your kitchen or bath, but they're still close to babysit for you.